THERMODYNAMIC SYSTEMS AND PROCESSES
Whenever one or more of the properties of a system change, a change in the state of the system
occurs. The path of the succession of states through which the system passes is called the
thermodynamic process. One example of a thermodynamic process is increasing the temperature
of a fluid while maintaining a constant pressure. Another example is increasing the pressure of
a confined gas while maintaining a constant temperature. Thermodynamic processes will be
discussed in more detail in later chapters.
When a system in a given initial state goes through a number of different changes in state (going
through various processes) and finally returns to its initial values, the system has undergone a
cyclic process or cycle. Therefore, at the conclusion of a cycle, all the properties have the same
value they had at the beginning. Steam (water) that circulates through a closed cooling loop
undergoes a cycle.
A reversible process for a system is defined as a process that, once having taken place, can be
reversed, and in so doing leaves no change in either the system or surroundings. In other words
the system and surroundings are returned to their original condition before the process took place.
In reality, there are no truly reversible processes; however, for analysis purposes, one uses
reversible to make the analysis simpler, and to determine maximum theoretical efficiencies.
Therefore, the reversible process is an appropriate starting point on which to base engineering
study and calculation.
Although the reversible process can be approximated, it can never be matched by real processes.
One way to make real processes approximate reversible process is to carry out the process in a
series of small or infinitesimal steps. For example, heat transfer may be considered reversible
if it occurs due to a small temperature difference between the system and its surroundings. For
example, transferring heat across a temperature difference of 0.00001 °F "appears" to be more
reversible than for transferring heat across a temperature difference of 100 °F. Therefore, by
cooling or heating the system in a number of infinitesamally small steps, we can approximate a
reversible process. Although not practical for real processes, this method is beneficial for
thermodynamic studies since the rate at which processes occur is not important.
An irreversible process is a process that cannot return both the system and the surroundings to
their original conditions. That is, the system and the surroundings would not return to their